The purpose is to provide information regarding the functional and anatomix maturation of the renin-angiotensin-aldosterone system. The renin-angiotensin system cannot be stimulated under 110 days gestation in the fetal lamb, and the response to furosemide increases with gestational age through the newborn period. Aldosterone cannot be stimulated by the renin-angiotensin system in the fetal lamb while it can in the newborn lamb. The pregnant mother appears to maintain aldosterone levels and therefore fluid and electrolyte balance in the fetal lamb. The newborn lamb has a hyperfunctioning renin-angiotensin-aldosterone system which is maintaining a relatively low blood pressure. When challenged with an angiotesin inhibitor the newborn lamb behaves similar to the adult with high renin hypertension. His blood presssre drops 15 mm Hg and plasma renin activity levels increase markedly. Vascular receptor sensitivity to angiotensin 2 will have to be tested in the newborn lamb. The renin-angiotensin system stimulates vasopressin in the newborn lamb, which may be important in dehydration, thirst, salt and wate balance, and blood pressure control.